docs: def interval_diff(progression1, progression2, interval):
      """Return the number of half steps progression2 needs to be diminished or
      augmented until the interval between progression1 and progression2 is
      interval."""
      i = numeral_intervals[numerals.index(progression1)]
      j = numeral_intervals[numerals.index(progression2)]
      acc = 0
      if j < i:
          j += 12
      while j - i > interval:
          acc -= 1
          j -= 1
      while j - i < interval:
          acc += 1
          j += 1
      return acc